1. Field of the Invention
The present invention relates to a display device having a dithering circuit and a gray scale processing system which performs gray scale processing on an input video signal.
2. Description of the Related Art
There is a known display device for displaying images which incorporates a gray scale processing circuit that performs gray scale processing on an input video signal to provide an increased number of pseudo-levels of gray scale.
For example, known as such a gray scale processing technique is a dithering technique in which four different dither coefficients “a” to “d” are each added to pixel data associated with each pixel in a set of four pixels that are adjacent to each other in the horizontal and vertical directions. For example, the dither coefficient “a” is added to the pixel data associated with the upper left pixel of the four pixels, the dither coefficient “b” is added to the pixel data associated with the upper right pixel, the dither coefficient “c” is added to the pixel data associated with the lower left pixel, and the dither coefficient “d” is added to the pixel data associated with the lower right pixel, respectively. However, in some cases, repeatedly adding the dither coefficients “a” to “d” to a screenful of pixel data in the aforementioned correspondence would cause a pseudo-pattern associated with the dither coefficients “a” to “d” to be perceived, i.e., a so-called dither noise would result.
In this context, another dithering technique was suggested in which the assignment of the dither coefficients “a” to “d” each to be added to pixel data associated with each of the four pixels adjacent to each other is changed for each field of an input video signal (e.g., see FIG. 8 in Japanese Patent Kokai No. 2001-312244; Patent Document 1). However, an addition operation according to this dithering technique may cause the average brightness level over the entire screen to vary from field to field, thus producing flicker.
A conventionally known display device for displaying images comprises a gray scale processing circuit which performs gray scale processing on an input video signal by error diffusion and dithering to provide an increased number of brightness levels for an image to be displayed on the screen (e.g., see FIGS. 24 to 27 in Japanese Patent Kokai No. 2000-227778; Patent Document 2).
In the error diffusion, for example, the input video signal is first converted to 8-bit pixel data associated with each pixel of the display, with the six high order bits being interpreted as display data and the remaining two least significant bits being interpreted as error data. Each error data of the aforementioned pixel data associated with each surrounding pixel is assigned a weight and added, thus allowing the resulting data to be reflected on the aforementioned display data. Such an operation allows the pseudo-brightness of the original pixel commensurate with the two least significant bits to be represented by the aforementioned surrounding pixels. Therefore, the display data of 6 bits being less than 8 bits makes it possible to represent the same levels of brightness as the aforementioned pixel data of 8 bits. The error diffusion pixel data of 6 bits obtained through the error diffusion is subjected to dithering.
In the dithering, a plurality of pixels adjacent to each other is defined as one pixel unit, and then different dither coefficients are each assigned to the aforementioned error diffusion pixel data associated with each pixel in the one pixel unit and then added, thereby providing dither added pixel data. In terms of the aforementioned one pixel unit, such an addition of the dither coefficients makes it possible to provide image data that enables only the four high order bits of the aforementioned dither added pixel data to represent the brightness corresponding to the 8 bits.
However, in some cases, the error diffusion and dithering performed on the input video signal as described above would result in a problem such as flicker.